fix(developer): Add Wordlist button now copies into source folder

Fixes #2185.

If a wordlist file is not in the same folder as the .model.ts file,
Keyman Developer will now ask the user if they want it copied into the
folder.

Also minor fix: if the user deletes a row in a wordlist, it marks the
file as modified.

Also very minor: the initial folder for the Add Wordlist dialog is now
the same as the .model.ts file.